In Snow Leopard's Preview, if you Control– or Right-click on a PDF and change the display option (e.g. from Single Page Continuous to Two Pages), this setting does not stick between launches. The next time you open a PDF, Preview reverts back to the original display setting.

However, if you change the display setting from the menu (View » PDF Display) the setting does stick, and becomes the new default for viewing PDFs in Preview.